1. Field of the Invention
This invention relates to digital radio systems, and more specifically, to demodulation of a transmitted signal and synchronization between the transmitter and receiver of digital radio systems.
2. Description of Related Art
The U.S. digital cellular telephone system uses time division multiple access (TDMA) as the channel access method. In this system, typically 3 to 6 users (data channels) share a common 30 KHz channel. Each user transmits data in an assigned time slot that is a part of a larger frame. Typically the gross bit rate of the data to be transmitted over the mobile channel is 48.6 kilobits per second (kbps). The modulation method is .pi./4 shifted-Differentially encoded Quadrature Phase Shift Keying (DQPSK). A typical system uses a Square Root Raised Cosine transmit pulse shape for a transmission signal with a roll-off of 0.35, or an excess bandwidth of 35% to shape the transmitted data. A filter having an impulse response matched to the transmit pulse shape is used in the receiver. Coherent detection, differential detection, or discriminator detection may be used to demodulate signals encoded by .pi./4-shifted-DQPSK techniques.
Disturbances due to multipath propagation affect the digital cellular transmission and require the use of an equalizer in the receiver. Equalizers employ coherent demodulation and are complex to implement. With the current technology, the complexity of an equalizer makes its use in portable radio receivers impractical. For such applications a digital discriminator or a differential detector is more suitable. These receivers are simple and can be implemented on a commercial digital signal processor (DSP), which forms the core of a modern digital mobile radio receiver.
When transmission introduces echoes in the received signal, called multipath propagation, the bit error rate (BER) performance of these detectors degrades very rapidly. Multipath propagation manifests itself in dispersion of the transmitted signal. This dispersion is characterized by a quantity called delay spread. Delay spread can be defined as the time interval between the first arriving signal and last significant echo. However, in most regions of cellular operation in the U.S., the root mean square (RMS) delay spread will not exceed 14 microseconds or approximately 34% of a symbol duration at this transmission rate (see, T. S. Rappaport, S. Y. Seidel and R. Singh, "900 MHz Multipath Propagation Measurements for U.S. Digital Cellular Radiotelephone", IEEE GIobecom Conference Record, Vol. 1, pp. 3.2.1-3.2.6, Nov. 1989, Dallas, Tex.) and hence degradation can be expected to be within acceptable limits.
The conventional discriminator comprises the following sequence of processing functions: limiting, differentiation, envelope detection, and integration. Specifically, after the RF signal is received, it is heterodyned down by conventional methods to an intermediate frequency (IF). The signal is then limited by normalizing its amplitude to a constant value with a limiter. A differentiator and an envelope detector convert frequency to a signal voltage. This type of discriminator can also be used to demodulate analog FM and digital .pi./4-shifted-DQPSK signals (see, "Discussion of .pi./4-Shifted-DQPSK", TR45.3.3/89.3.14.5, submitted to TIA Technical Subcommittee TR45.3, WG III, Modulation Task Group, Mar. 14, 1989, available from the Electronics Industries Assoc., Engineering Dept., 2001 Eye Street, N.W., Washington, D.C. 20006). A problem with the above conventional discriminator is the presence of the limiter. The limiter makes the discriminator sensitive to the pulse shaping roll-off factor of the transmitted pulse signal. An increase in BER occurs by reducing the roll-off factors or by reducing the excess bandwidth of the transmitted signal. This puts a restriction on the data rate and the filtering in the system.
A second problem is the recovery of symbol timing and carrier frequency error correction. Typically, these conventional discriminators require additional circuits having phase locked loops (PLLs) to perform these functions.
It would be advantageous to create a simplified detector that does not require a limiter, but is able to recover from symbol timing and correct carrier frequency errors.